1. Field Of The Invention
This invention relates to process sequences for the reduction of fouling in the fractional distillation of light end hydrocarbon components, such as those produced by catalytic cracking, pyrolysis or steam cracking. More particularly, the invention relates to process sequences to reduce fouling by use of upstream hydrogenation unit configurations, rather than the multiple hydrogenation unit configurations used in conventional fractional distillation systems.
2. Background
Steam crackers can operate on light paraffin feeds such as ethane and propane, or on feedstocks which contain propane and heavier compounds to make olefins. Steam cracking these heavier feedstocks produces many marketable products, notably propylene, isobutylene, butadiene, amylene and pyrolytic gasoline.
In addition to the foregoing, small quantities of undesirable contaminants, such as di- and poly-olefins, and acetylinic compounds are produced. These contaminants may also be produced with olefins from catalytic cracking. These contaminants may cause equipment fouling, interfere with polymerization reactions, and in some cases pose safety hazards. It is, therefore, highly desirable to remove them from the cracked stream in the downstream recovery process.
The recovery of the various olefin products from either type of cracked stream is usually carried out by fractional distillation using a series of distillation steps or columns to separate out the various components. The unit which separates hydrocarbons with one carbon atom (C.sub.1) and lighter fraction is referred to as the "demethanizer". The unit which separates hydrocarbons from the heavier components with two carbon atoms (C.sub.2) from the heavier components is referred to as the "deethanizer". The unit which separates the hydrocarbon fraction with three carbon atoms (C.sub.3) from the heavier components is referred to as the "depropanizer". The unit which separates the hydrocarbon fraction with four carbon atoms (C.sub.4) is referred to as the "debutanizer." The residual heavier components having a higher carbon number fraction (C.sub.5 +) may be used as gasoline or recycled back to into the steam cracker. The various fractionation units may be arranged in a variety of sequences in order to provide desired results based upon various feedstocks. To that end, a sequence which uses the demethanizer first is commonly referred to as the "front-end demeth" sequence. Similarly, when the demethanizer is used first, it is commonly referred to as the "front-end deeth" sequence. And, when the depropanizer is used first, it is commonly referred to as "front-end deprop" sequence.
In all of the sequences, the gases leaving the steam cracker are quenched and have their acid gas removed. At this point, the various flow sequences diverge. In the conventional front-end demethanizer sequence, as illustrated in FIG. 2, the quenched and acid-free gases containing hydrocarbons having one to five or more carbon atoms per molecule (C.sub.1 to C.sub.5 +) first enter a demethanizer, where hydrogen and C.sub.1 are removed. This tower operates at very cold temperatures (ie.-300.degree. C.) and therefore has a reduced tendency to foul. The heavy ends exiting the demethanizer, consists of C.sub.2 to C.sub.5 + molecules. These heavy ends then are routed to a deethanizer where the C.sub.2 components are taken over the top and the C.sub.3 to C.sub.5 + compounds leave as bottoms. The C.sub.2 components leaving the top of the deethanizer are fed to an acetylene converter and onto a C.sub.2 splitter which produces ethylene as the light product and ethane as the heavy product. The C.sub.3 to C.sub.5 + stream leaving the bottom of the deethanizer is routed to a depropanizer, which sends the C.sub.3 components overhead and the C.sub.4 to C.sub.5 + components below. The C.sub.3 product may be hydrotreated to remove C.sub.3 acetylene and diene before being fed to a C.sub.3 splitter, where it is separated into propylene at the top and propane at the bottom, while the C.sub.4 to C.sub.5 + stream is fed to a debutanizer, which produces C.sub.4 components at the top with the balance of C.sub.5 + components leaving as bottoms to be used for gasoline or to be recirculated into the furnace or cracker as feedstock. Both the C.sub.4 and the C.sub.5 + streams may be separately hydrotreated to remove undesirable acetylenes and dienes.
In conventional front-end deethanizer sequences, as illustrated in FIG. 3, the quenched and acid free gases containing C.sub.1 to C.sub.5 + components first enter a deethanizer. The light ends exiting the deethanizer consist of C.sub.2 and C.sub.1 components along with any hydrogen. These light ends are fed to a demethanizer where the hydrogen and C.sub.1 are removed as light ends and the C.sub.2 components are removed as heavy ends. The C.sub.2 stream leaving the bottom of the demethanizer is fed to an acetylene converter and then to a C.sub.2 splitter which produces ethylene as the light product and ethane as the heavy product. The heavy ends exiting the deethanizer which consist of C.sub.3 to C.sub.5 + components are routed to a depropanizer which sends the C.sub.3 components overhead and the C.sub.4 to C.sub.5 + components below. The C.sub.3 product is fed to a C.sub.3 splitter where it is separated into propylene at the top and propane at the bottom, while the C.sub.4 to C.sub.5 + stream is fed to a debutanizer which produces C.sub.4 compounds at the top with the balance leaving as bottoms to be used for gasoline or to be recirculated. As with the demethanizer sequence, the C.sub.3, C.sub.4, and C.sub.5 + streams may separately hydrotreated to remove undesirable acetylenes and dienes.
In conventional front-end depropanizer sequences, as illustrated in FIG. 4, the quenched and acid-free gases containing hydrocarbons having from one to five or more carbon atoms per molecule (C.sub.1 to C.sub.5 +) first enter a depropanizer. The heavy ends exiting the depropanizer consist of C.sub.4 to C.sub.5 + components. These are routed to a debutanizer where the C.sub.4 's and lighter species are taken over the top with the rest of the feed leaving as bottoms which can be used for gasoline or other chemical recovery. These steams may be separately hydrotreated to remove undesired acethylenes and dienes. The tops of the depropanizer, containing C.sub.1 to C.sub.3 components, are fed to an acetylene converter and then to a demethanizer system, where the C.sub.1 components and any remaining hydrogen are removed as an overhead. The heavy ends exiting the demethanizer system, which contains C.sub.2 and C.sub.3 components, are introduced into a deethanizer wherein C.sub.2 components are taken off the top and C.sub.3 compounds are taken from the bottom. The C.sub.2 components are, in turn, fed to a C.sub.2 splitter which produces ethylene as the light product and ethane as the heavy product. The C.sub.3 stream is fed to a C.sub.3 splitter which separates the C.sub.3 species, sending propylene to the top and propane to the bottom.
In conventional distillation sequences, as described above, multiple hydrogenation units are used to remove contaminants. The location and complexity of a typical hydrogenation unit is set by the compatibility of process conditions with the catalyst system used and the products being treated. Hydrogenation units required for the production of the aforementioned marketable distillation products include, in addition to the acetylene converter which treats the C.sub.2 stream, a methylacetylene/propadiene converter ahead of the C.sub.3 splitter to remove contaminants from propylene and propane products and to avoid the risk of detonation in the C.sub.3 splitter caused by build-up of methylacetylene and propadiene, a hydrogenation unit ahead of the debutanizer to remove C.sub.4 and C.sub.5 acetylenes from C.sub.4 and C.sub.5 olefins, and either a heat soaker or a hydrogenation unit on the debutanizer bottoms to remove additional C.sub.5 acetylenes from pyrolysis gasoline. There is, therefore, a requirement of multiple, separate and distinct hydrogenation units. While such a configuration is generally effective to remove contaminants, it is costly. The hydrogenation units required in this configuration are often very similar in nature and often require large recycle loops to moderate the reaction and fractionation facilities to remove excess hydrogen and other gases. Furthermore, since the hydrogenation units are downstream of most the equipment in a steam cracker facility, the equipment, including fractionators, boilers and pumps, are often subject to costly fouling due to the presence of undesired contaminants.
It would be desirable if one could develop a treatment method for fractionating the C.sub.2, C.sub.3 and C.sub.4 hydrocarbon components from a steam cracked hydrocarbon stream which eliminates or reduces fouling in the fractionation units caused by di-olefinically and acetylinically unsaturated hydrocarbon contaminants in the stream without unduly complicating the process sequence or increasing the capital and processing costs of the operation.